1. Field of the Invention
The present invention is related to the field of devices used in networks, and more specifically to network nodes that verify the power configuration of a device before supplying power to it.
2. Description of the Related Art
Integrated Services Local Area Networks (ISLANs) include nodes such as switches, gateways, routers, bridges, repeaters, etc. Network nodes are referred to generally below as network switches, but include any device used for processing information in a network. Endpoints connected to the network, such as Internet phones, can receive power along with data from the network switch. Such endpoints are called line powered devices.
An example of a line powered device is described with reference to FIG. 1. A network node, such as ISLAN Access Unit (AU) 12, is connected to two devices 14 and 16. Device 14 is a line powered telephone. Device 14 is connected to Port A of ISLAN AU 12 by a cable 14a. Switch 12 applies power remotely to device 14. Applied power is depicted by voltage VN traveling over cable 14a. 
A serious problem exists when network switch 12 inadvertently applies power to a non-line powered device. In the example of FIG. 1, device 16 is unsuitable for remote line powering. Device 16 includes a network card 18 connected to Port B of ISLAN AU 12 by a cable 16a. Card 18 is not configured for receiving remote power from switch 12. If switch 12 applies power over cable 16a, the incoming waveform VN can damage card 18, as shown by a flash 18a. 
In conventional circuit switched networks, telephones are often powered through the telephone lines. However, the telephones in packet switched networks may or may not be powered remotely by the network switch 12 over the network cable. In packet switched networks, the switch can detect a device coupled to a port (assuming the device already has power), but does not know whether the device is suitable for remote powering before power is applied to the device. What aggravates the problem is that such devices are often connected to the network switch at remote locations. For example, telephones are typically powered from a central location (e.g. a wiring closet) instead of locally (e.g. in the same office). This makes it hard to configure the network for different line powered devices.
The problem is worsened in the event of loss of power. In the case of telephones, users have come to expect service under all conditions, including loss of power. The device must therefore operate without interruption when local power is not available.
Accordingly, a need exists for identifying line powered devices on a packet switched network.
The present invention ascertains the suitability for remote powering a device from a network switch, before the network switch supplies power to the device. The network switch determines suitability by interrogating all coupled devices.
The switch sends a discovery signal to the ports connected to remote devices and monitors each port for a return signal. If a return signal is received, it is compared to the transmitted discovery signal. If the signals are identical, accounting for the expected signal losses in the wire, the coupled device is deemed suitable for receiving power remotely from the network switch. Accordingly, the network switch then supplies power to the line powered devices. If the network switch fails to discover any device, the switch periodically retries this discovery signal in order to detect new devices that are connected. The network switch also monitors ports that have power applied, looking for loss of signal, indicating that a device has been detached. Power then needs to be removed before another device is connected.
The invention eliminates having to preprogram the network switch with the type of devices connected to each port. This reduces the amount of time required to initially program the switch, and also eliminates having to reprogram the switch every time a new device is connected. The invention also eliminates having to reprogram the switch after loss of power. The invention is particularly useful in network locations without local power, or with a central power distribution facility that uses an uninterrupted power supply.